Steel is one of the most ubiquitous structural materials in the world. Unfortunately, steel is prone to oxidation and thereby structural and decorative failure. Many techniques have been developed to attempt to provide a protective coating for steel; these include galvanizing, galvannealing, chromizing, cladding, painting and the like.
A good method for protecting steel is providing a stainless steel composition on the exterior of the steel product. Chromizing is a common method for the production of chromium-iron alloys, thereby stainless steels, on the surface of steels. Chromizing steel involves thermal deposition-diffusion processes whereby chromium is diffused into the steel and produces a varying concentration of chromium in the steel substrate. Typically, the surface of the substrate has the highest chromium concentration and as the distance into the substrate increases the chromium concentration falls off. Often the chromium concentration follows a typical diffusion function, that is, the chromium concentration decreases exponentially as a function of distance from the substrate. Other chromizing products, such as that described in U.S. Pat. No. 3,312,546, include diffusion coatings that have chromium concentrations above 20% that decrease linearly as a function of distance into the substrate (see FIG. 1). These high chromium-content coatings appear to include a foil or layer of chromium containing material carried by the bulk substrate.
The decreasing concentration of chromium as a function of depth into the substrate can affect the corrosion resistance of the material. That is, abrasion of the surface continuously produces new layers with lower chromium concentrations that are well understood to be less corrosion resistant than the initial surface. This undesirable effect, due to the variable concentration of chromium in the chromized surfaces, has been overcome by the advent of explosive cladding.
Explosive welding or cladding of stainless steel onto a carbon steel produces a stainless steel layer with a consistent composition metallurgically bonded to a carbon steel substrate. This technique overcomes the variable concentrations associated with chromizing but is severely limited by the thicknesses of the flying layer, the use of high explosives, and the metallurgical bond that is formed. Two types of metallurgical bonds have been observed in explosively welding metals: under high explosive loading the cross-section to be composed of a wave-like intermixing of the base and flying layers and under lower explosive loadings the cross-section includes an implantation of grains of the flying layer into the base layer. E.g. see: Explosive welding of stainless steel—carbon steel coaxial pipes, J. Mat. Sci., 2012, 47-2, 685-695; and Microstructure of Austenitic stainless Steel Explosively Bonded to low Carbon-Steel, J. Electron Microsc. (Tokyo), 1973, 22-1, 13-18.
The prior art fails to teach a material that includes a stainless steel layer with a consistent composition diffusion bonded to a carbon steel substrate. Ideally, such a material would include the corrosion resistance associated with the explosively welded stainless steel and the deep diffusion bonding observed typical of chromizing applications.